Pre-Conditioning Leakage Current

We are following the 60601-1 standard and were performing the leakage current test after pre condition the unit for 7 days @ RH 93. One of our pieces of equipment is no longer working could we replace this board and run the leakage current or fix whatever the problem on that board is and run the leakage currents?

In the requirement it says to wait 1hour after preconditioning but what if we wait 12 hours is the test still valid why or why not?

john95 said:

We are following the 60601-1 standard and were performing the leakage current test after pre condition the unit for 7 days @ RH 93. One of our pieces of equipment is no longer working could we replace this board and run the leakage current or fix whatever the problem on that board is and run the leakage currents?

In the requirement it says to wait 1hour after preconditioning but what if we wait 12 hours is the test still valid why or why not?

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If you don't follow the requirements, the test is not valid. Obviously the test is dependent upon the humidity or moisture absorbed by the UUT as well as other conditions. Changing the board with one that hasn't been subjected to the required humidity would invalidate the test because it will not have an opportunity to absorb the moisture. Waiting a longer period after preconditioning than required will allow the moisture to evaporate from the UUT. Evaporate means to draw off moisture. Evaporation

It is worth to note that most modern equipment does not use hygroscopic insulation (material which absorbs moisture) in parts providing safety insulation. There are some unusual cases (e.g. powder insulated heaters, impregnated paper) where the test is important.

It is also worth to note that most EMC capacitors have a negative temperature co-efficient. In some cases the leakage can be up to 20% higher in cold condition due to this effect. It has nothing to do with the equipment absorbing moisture. So even if you don't run the humidity treatment, make sure to measure the leakage in cold condition.

Have a look at the parts and materials forming safety insulation, and decide if there is any hygroscopic material.

Peter Selvey said:

It is worth to note that most modern equipment does not use hygroscopic insulation (material which absorbs moisture) in parts providing safety insulation. There are some unusual cases (e.g. powder insulated heaters, impregnated paper) where the test is important.

It is also worth to note that most EMC capacitors have a negative temperature co-efficient. In some cases the leakage can be up to 20% higher in cold condition due to this effect. It has nothing to do with the equipment absorbing moisture. So even if you don't run the humidity treatment, make sure to measure the leakage in cold condition.

Have a look at the parts and materials forming safety insulation, and decide if there is any hygroscopic material.

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My opinion is little bit difference. Firstly, the insulation resistance of capacitor has negative temperature co-efficient, rather than capacitance. If you say capacitor with negative temperature coefficient, that means that the capacitance decreases with increasing temperature.

Secondly, I do not believe that the humidity treatment specified in IEC 60601-1 can influence the EMC capacitor. EMC capacitors shall comply with one of the subclasses of IEC 60384-14 (such as, Y1 or Y2) and those capacitors shall suffer the damp heat, steady state test as specified in IEC 60384-14 (such as, 40+/-2 degree, 93%+/-3%RH, 21 days).

Thirdly, I do not believe that the humidity treatment test intended to focus on the temperture. The temperature of the air in the humidity chamber can be any value in the range of 20 degree to 32 degree. That said, only ambient temperature (that is COLD) is expected.

I agree with that most modern equipment does not use hygroscopic insulation. However, the high humidity will increase the leakage current of insulation surface, since water film formed on the surface of insulation.

No exception can be made from the requirement of humidity treatment. However, the test shall be applied only to those equipment parts likely to create a safety hazard when influenced by the climatic conditions that are simulated by the test.

The bottom line is that you must perform the test as stipulated in the standard.

Just my opinion of course:

I agree we should follow the standard, but the standard says only apply to parts that are influenced by humidity (both 2nd and 3rd edition). There are many test labs that do not apply the humidity test with a justification of no hygroscopic materials. Also many test lab auditors complain that the number of test samples in humidity chambers during an audit never seems to match the number expected (implying that many test labs skip the test even though documenting it is done). There are of course many labs that do it all the time.

The negative temperature co-efficient means that capacitance decreases with temperature, which means leakage current also decreases (meaning the maximum is in cold condition). This effect has been observed in tests over 15-20 years, verified by simulation (heating up an EMC capacitor) and also confirmed by inspection of datasheets for capacitors of popular manufacturers. The extent of the effect depends on the position of the capacitors to hot parts, and also the quality, with higher quality caps having little change with temperature.

The insulation resistance of capacitors has negligible effect on leakage currents (normally 100Mohm+ >> capacitive impedance of 2-3Mohm). If the insulation resistance reduced to a point where it impacts leakage current tests, it would generate enough heat to destroy the component during dielectric strength tests.

Finally, it is wrong to consider that water on the surface of insulation has any impact on the test. Dielectric strength is a test for solid insulation only and not intended as a test of the creepage distance over surfaces. The test is also required to be done carefully to avoid any surface film of water (i.e. condensation caused by a sudden drop in temperature). If a breakdown was found to be caused surface water film it is not a test failure (i.e. not dielectric breakdown). The only valid failure is one where the solid insulation absorbed water leading to breakdown.

If as you said, I will be very surprised.

All of our products have higher leakage current after humidity treatment (about 10% higher than the values in cold condition). But I can not find any hygroscopic insulation has been employed in our equipments. Did the CB test lab tell a lie and issue a wrong CB report? BTW, our company has been established nearly 15 years, I am working for the R&D department.

In fact, the capacitors have positive or negative temperature coefficient, it depends. Anyway, the humidity treatment test almost has nothing to do with the temperature (since ambient temperture is OK). Therefore, we talk about the temperature coefficient of components seem to be meaningless in humidity treatment test. Moisture is the major concern, rather than the temperature.

I agree with the point of insulation resistance of capacitor. Actually, I did not mean the insulation resistance would greatly influence the leakage current. The insulation resistance shall be not less than 100 MΩ according to IEC 60384 series. The contribution to the leakage current shall be microampere level. Again, the low temperature in humidity chamber would not affect the insulation resistance of capacitor.

I also agree that the condensation shall be avoided (this is also the requirement of the standard). But, more or less of water film is expected (In real world, it also shall be. We also did many in-house tests and found this phenomenon). I also did not mean that the water film will form a direct conducting channel and cause the insulation breakdown (I had rather thought, this would influence the distributed capacitance).

Actually, most modern equipments would not fail after the humidity treatment test. But, the leakage current indeed increase after the test. This contrary to your point of view. I think the practice is the criterion of truth.

If anything is wrong, please correct me.

I saw the same results as Ronald the leakage currents increased after preconditiong for 7 days.

p.s. i originally posted the topic but can't remember my login password.

Again, the increase in leakage current after humidity conditioning is most likely due to the negative temperature co-efficient (NTC) in EMC capacitors.

Try this experiment: allow the equipment to cool to room ambient (at least 4 hrs).

Record the earth leakage in cold condition. Then, allow the equpment to warm up, watching the earth leakage as it does. You should find the leakage current drops over time.

As before, the extent of the effect depends on location of hot parts with respect to the EMC capacitors and the quality of the capacitors (low cost = higher NTC).

Hi Steve,
The pre-conditioning and post-conditioning were both done at 23-24C +/- 1C. How much of a temperature difference do you think would cause a noticeable difference in leakage current?